It is important that you lock your computer when you leave it.
When you log on to your computer, you will have access to certain resources depending on who you are. If you leave your computer unlocked, other people can use it without your consent. This means that they may able to see and edit information in your name that only you should be able to see and edit.
If you are in doubt about how to lock your computer, you can find a manual below.
Here you can find two different ways to lock your Windows computer
If you use Linux without graphic user interface, remember to log out when you leave the computer.
If you use a graphic user interface, Linux is as standardised as Windows or Mac. Therefore, there can be different ways to lock your screen depending on distribution and some times depending on which user interface you use.
CTRL+ALT+L works in some distributions, however, the best advice is to use the help function which is build in to find the correct way for your system.